1. Field of the Invention
The present invention relates to a DC current sensor used in a DC leakage breaker and the like, and, particularly, it relates to a highly versatile DC current sensor, having a relatively simple construction and a good detecting capability against microscopic current variation, which can be mounted very easily to a wired lead being detected for realizing a stable detection.
2. Description of the Prior Art
In recent years, though equipments using a DC current such as electric appliances incorporating an inverter, electromobiles and the like are increased, requirements on a sensor which detects load of DC motors built in the various equipments for necessary control, and a DC current sensor used in a DC leakage breaker and the like are enhanced.
As the current sensor used in an AC leakage breaker and the like, those applying a current transformer is widely known.
However, this configuration can not be adopted in the leakage breaker used in the aforementioned equipments using the DC current, and hence, a conventionally known shunt resistance method, magnetic amplifier method, magnetic multi-vibrator method (Japanese Patent Application Laid Open Nos. Sho 47-1644, Sho 53-31176, Sho 59-46859), hall device and the like have been studied for utilization.
The shunt resistance method is a method for detecting a potential difference produced at opposite end portions of the shunt resistance which is disposed in series in the lead wire being detected.
In the magnetic amplifier method and the magnetic multi-vibrator method, a core made of a soft magnetic material constituted by winding a detecting coil in a toroidal shape is used, wherein the lead wire being detected is extended through the core for DC magnetic deflection of the core made of the soft magnetic material, within a saturated magnetic flux density (Bs), by the DC current flowing through the lead wire being detected, thereby imbalance is produced in a time the alternating magnetic flux produced by applying the AC current to a coil wound on the core in advance reaching saturation in the positive and negative direction, to detect the variation by the detecting coil. Though the magnetic amplifier method is constituted such that, in order to give the magnetic flux variation in the core in advance, an exciting coil is wound on the core to apply the AC current having a predetermined value, the magnetic multi-vibrator method is constituted such that, the self-oscillation takes place by the operation of a semiconductor and the like in a circuit connected to the detecting coil, and a duty ratio of the oscillation waveform is changed responsive to the electric current being detected for oscillation.
Furthermore, the hall device method is constituted such that, the lead wire being detected is wound in a toroidal shape directly around the core made of soft magnetic material, whereon a gap portion for disposing the hall device is formed partly, and the magnetic flux variation in the core responsive to the variation in the DC current flowing through the lead wire being detected is detected directly by the hall device.
However, the DC current sensors consisting of the above-mentioned methods are difficult to respond to a microscopic current variation of the DC leakage breaker and the like by the following reasons, and at present, they are not practically used as the sensitive DC current sensor.
That is, in the shunt resistance method, since the shunt resistance itself is disposed, as an electrical resistance, in a circuit including the lead wire being detected, an electrical loss in the circuit increases and an electrical efficiency is deteriorated.
Likewise, since a detecting circuit for detecting the potential difference produced at opposite ends of the electrical resistance is directly connected to the lead wire being detected, the detecting circuit and the lead wire being detected are difficult to be insulated electrically, and for example, the detecting circuit and an applied circuit such as a microcomputer control circuit, and the like can not be connected directly, results in a poor versatility.
Besides they disadvantages, though two shunt resistances must be disposed in the circuit of the lead wire being detected in order to adopt the shunt resistance method in the leakage breaker, it is, in fact, difficult to make characteristics of the respective shunt resistances uniform, thus a precise measurement of the potential difference can not be realized.
Besides, in order to detect a small leakage by comparing the potential difference measured by the detecting circuit connected to the respective shunt resistances, it is necessary to connect the respective detecting circuits through a very complicated electric circuit, thus it is difficult to provide as the highly practical DC current sensor.
In the magnetic amplifier method and the magnetic multi-vibrator method, though the detecting circuit and the lead wire being detected can be insulated electrically, as previously described, the core made of the soft magnetic material must be magnetically deflected by the DC current flowing through the lead wire being detected, so as to be saturated substantially near to the saturated magnetic flux density (Bs). When using the known soft magnetic material such as a permalloy as the core, and for example, the electric current flowing through the lead wire being detected is about several tens of mA, the lead wire being detected must be wound around the soft magnetic material core by several tens to several hundreds of turns or more, thus, originally, it was difficult to be used as the DC current sensor for the leakage breaker and the like requiring one through turn of the lead wire being detected.
Also in the hall device method, since the detecting capability is inevitably decided by characteristics of the hall device, when the hall device known at present is used, and for example, the electric current flowing through the lead wire being detected is about several tens of mA, the lead wire being detected must be wound around the soft magnetic material core above several hundreds to several thousands of turns, thus, similar to the magnetic amplifier method an the magnetic multi-vibrator method, it was difficult to be used as the DC current sensor for the leakage breaker and the like requiring one through turn of the lead wire being detected.